A laser (acronym for light amplification by stimulated emission of radiation) emits coherent light through a process of optical amplification via the stimulated emission of electromagnetic radiation. Lasers or coherent light sources have many important applications which form the cornerstones of our modern society. These range from fast information processing and telecommunications; optical data storage; bio-imaging; medical diagnostic and phototherapy to scientific research and defence applications. The demands for such applications continue to grow with Mankind's relentless pursuit of sustainable growth.
The heart of a laser is its gain medium—a material that makes light stronger or permits optical amplification to occur. Optical amplification, also known as gain occurs when the gain material transfers part of its energy to light and makes the light more intense and in phase. Typical gain media include crystals (e.g., neodymium-doped yttrium aluminum garnet (Nd:YAG), titanium doped aluminum oxide (Ti:Sapphire) and neodymium-doped yttrium orthovanadate (Nd:YVO4) etc.) and high quality semiconductors (e.g. gallium arsenide (GaAs) and aluminum gallium arsenide (AlxGa(1-x)As) etc). These gain media typically operate in the infrared (IR) region of the electromagnetic spectrum. To obtain wavelengths in the ultraviolet (UV) or visible (VIS) region, wavelength conversion of IR photons with nonlinear crystals is performed. For example, in the modest hand-held laser pointer, the green light is generated indirectly—beginning with an AlGaAs laser diode (808 nm) pumping a NdYVO4 crystal to generate 1064 nm photons which are then frequency doubled by a KTP crystal to 532 nm. It is important to note the stringent conditions needed to prepare the high quality, crystalline gain materials, which involve costly high temperature growth and processing. For example, GaAs and AlxGa(1-x)As heterostructures require expensive elevated temperature and high vacuum growth techniques such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE).